The invention pertains to the field of stereotactic radio surgery and particularly relates to a system for this purpose using a rotating array of radiation sources.
Therapeutic use of a flux of high energy photons dates from the early work of Roentgen. The object of the therapy is the destruction of a specifically defined volume of living tissue while sparing adjacent tissue. The development of method and apparatus for this purpose has generated a voluminous literature. In a more recent progression in this field, there has developed the use of plural radiation sources directed at the treatment voxel from diverse directions. Very high precision of radiation dose and dose distribution is achievable from this stereotactic approach. The treatment volumina typically range from 20 cm3 to 0.5 cm3 and precision direction of the radiation beam is required. Included among applications of this treatment are cancer metastasis, primary cancer of the brain, arterial-venous malformations, certain types of: facial pain, epilepsy, Alzheimer""s disease, and Parkinson""s disease.
One system of the prior art is the Leksell GamraKnife(trademark). This system includes a semi-spherical source body containing a number of radiation sources enclosed by a thick radiation shield. The source body has open beam channels directed from the individual source locations radially inwards toward a common focal point. This is described in European Patent Publication EP-248774. In such a system, each of the multiple radiation sources provides only a small dose to intervening tissue, with the resulting maximum radiation dose available only at the common focal locus where the several radiation beams intersect. Accordingly, living tissue at the focal locus will be effectively killed without substantial harm to surrounding tissue after treatment for a prescribed time. Thus, this system of the prior art uses a great many radiation sources, as for example 20160Co sources of equal activity, regularly arranged on a hemisphere. With the increasing number of sources of less activity, the relative radiation exposure of healthy tissue is substantially decreased relative to the tissue at the focal volume. Such a construction requires a corresponding increase of heavy shielding material resulting in considerable weight and volume. It is also necessary to achieve an orderly and accurate array of hundreds of bores in the structure, arranged on the surface of a hemisphere of large diameter and defining a common focal point with precision. The manufacture of this equipment is a difficult and expensive exercise. Moreover, this static arrangement of many radioactive sources leads to a very high cost in the procurement and handling of many sources which must exhibit a high uniformity in specific activity. Fewer radiation sources would be preferable in such a stereotactic system.
Another prior art system is disclosed in U.S. Pat. Nos. 5,528,653 and 5,757,886, each assigned to Song. In this system, 30 sources are uniformly distributed (azimuthally) around a half sphere which is adapted to rotate about its symmetry axis during patient treatment. The rotational axis coincides roughly with the longitudinal axis of the patient. This prior art system further includes a concentric collimator hemisphere disposed within the source body hemisphere. This collimator is adapted to be rotated independently of the source body hemisphere to align a desired size aperture with the corresponding source. The collimator is then locked to the source carrier hemisphere and the rotation of the source collimator assembly allows stereotacticaly irradiating the patient with selected size radiation beams.
A rotating symmetrical source system as described is disadvantaged compared to a static system as described in that individual sources or groups of sources can not be blocked is independently from each other. Such selective blocking is desired or necessary, for example to avoid irradiation of specifically sensitive tissue adjacent the treated volume.
Both the static and rotating systems of prior art are further deficient or undesirable in their large size, weight and moment of inertia; inapplicability for body parts other than the head; claustrophobic effect of the treatment chamber; need for shielding doors to reduce radiation levels outside the treatment chamber; loss of focal precision due to one sided cantilever bearing of the rotating mechanism; and, absence of a preferred standby or service mode shielding position. Moreover, the prior art is not adaptable to dynamic modulation of the collimated flux during a treatment.
The present invention implements a novel design for a stereotactic radio surgery system wherein the irradiation facility, in one geometry, comprises an open structure requiring minimal shielding while facilitating radiation treatment locations other than the head. This is achieved by relationship of axial and radial dimensions for the patient acceptance region and the mutual arrangement of an asymmetric rotating source carrier of limited angular interval diametrically opposite a beam catcher and counterweight structure adapted to minimize scattered radiation. The source array has the form of an oblique two dimensional array of source locations which allows for a selected pattern of sources of selectable collimation and strengths. The collimation function is obtained with a collimator structure composed of annular collimator rings, adapted for independent relative rotation, each capable of alignment with an approximated one dimensional sub-array of the source array. A variety of aperture sizes are available by rotation of a collimator ring with respect to the corresponding source sub-array, after which the collimator ring(s) synchronize with the source subarray(s) for irradiation of the designated tissue volume.
The independent synchronous rotation of source array and collimator structure is maintained to a high degree of precision and a controlled relative rotation for a short period of time enabling alignment of a different collimator array with the source array during a specified portion of a revolution. Thus the controlled independence of the rotating source and collimator members affords dynamic modulation capability for treatment dose.
Apart from reduction of the weight of shielding, the open structure of this geometry permits superposing the focal locus on body regions of the patient other than the head.
Another geometry which consists of a half closed embodiment particularly suited to cranial treatment employs an asymmetric array of sources disposed within a limited sector of a spherical shell. A set of collimator arrays is selectable on an inner collimating structure rotating coaxially with the source array sector.